1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention relates to a method for manufacturing the photoreceptor, and to an electrophotographic image forming method and apparatus using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently the growth of electrophotographic information processing apparatus (hereinafter image forming apparatus) such as copiers, printers and facsimiles is remarkable. In particular, photo-printers capable of recording digital information using light have been drastically improving in recording qualities and reliability. This digital recording technique is applied to copiers as well as photo-printers. The digital copiers to which this digital technique is applied have various image forming functions. Therefore it is considered that the demand for the digital copiers increases more and more.
At the present time, laser diodes (LDs) and light emitting diodes (LEDs) are used as light sources for the photo-printers because of being small in size, and having a relatively low cost and good reliability. The LEDs which are typically used for the photo-printers emit light having a wavelength of 660 nm. The LDs which are typically used for the photo-printers emit near infrared light. Therefore, a need exists for a photoreceptor having high sensitivities over a wavelength range including the visible region and the near infrared region.
The wavelength range over which an electrophotographic photoreceptor has sensitivity almost depends on the wavelength range over which the charge generation material used in the photoreceptor has photosensitivity. Therefore, various kinds of materials, such as azo type pigments, polycyclic quinone type pigments, trigonal system selenium, phthalocyanine pigments and the like, have been developed for the charge generation materials.
Among these pigments, titanyl phthalocyanine pigments (hereinafter sometimes referred to as TiOPcs), which have been disclosed in Japanese Laid-Open Patent Publications Nos. (hereinafter JOPs) 3-35064, 3-35245, 3-37669, 3-269064 and 7-319179, are very useful as a photosensitive material for a photoreceptor used for electrophotographic image forming apparatus such as printers and copiers, which use an LED or LD as a light source. This is because titanyl phthalocyanine is sensitive to light having a relatively long wavelength of from 600 to 800 nm.
A photoreceptor used for electrophotography such as Carlson process and the like processes is required to have the following charge properties as well as the high sensitivity to the specific light mentioned above:
(1) good charging ability such that a high electric potential can be formed and maintained in a dark place;
(2) good charge decaying ability such that the electric potential previously formed on the photoreceptor rapidly decays and the residual potential is low when the photoreceptor is exposed to light; and
(3) good charge stability such that the photoreceptor can maintain a good charging ability and a good charge decaying ability even when the photoreceptor is used for a long time.
In particular, in high sensitive photoreceptors such as photoreceptors including a TiOPc, the charging ability tends to deteriorate and the residual potential tends to increase when the photoreceptors are repeatedly used. Namely, the photoreceptors including a TiOPc have an insufficient charge stability. Therefore, a need exists for a photoreceptor including a TiOPc, which has good charge stability.
The methods for synthesizing TiOPcs and the electrophotographic properties of the resultant TiOPc have been disclosed in JOPs Nos. 57-148745, 59-36254, 59-44054, 59-31965, 61-239248, 62-67094 etc. In addition, various crystal forms are known with respect to TiOPcs, and JOPs Nos. 59-49544, 59-166959, 61-239248, 62-67094, 63-366, 63-116158, 63-196067, 64-17066 etc. have disclosed TiOPcs having a different crystal form.
Among these TiOPcs, the TiOPc crystals having an X-ray diffraction spectrum such that a main diffraction peak is observed at a Bragg (2xcex8) angle of 27.2xc2x0xc2x10.2xc2x0 when the crystals are exposed to the Cu-Kxcex1 X-ray have high photosensitivity particularly in a near infrared region. It is known that this TiOPc crystals have molecules of water therein as disclosed in Abstracts of the third meeting of Electrophotographic Technical Committee in 1991.
The TiOPcs mentioned above change their crystal forms when the water molecules release therefrom, resulting in changes of photosensitivity thereof. In attempting to avoid such changes of photosensitivity, JOP No. 04-338967 discloses a method in which water is included in a TiOPc when the TiOPc is synthesized. In addition, JOP No. 10-115940 discloses a photoreceptor including a charge generation layer including water therein.
On the other hand, halogen-containing solvents such as methylene chloride have been used for charge transport layer coating liquids because of having the following advantages:
(1) the coating liquids have good productivity;
(2) the resultant charge transport layer has good coating qualities;
(3) the resultant photoreceptor has good charging properties; and
(4) the solvents has a relatively low cost.
Halogen-containing solvents are not typically compatible with water. Therefore, the resultant photoreceptors tend not to cause such a problem as mentioned above.
However, currently it is considered that halogen-containing solvents adversely affect the natural environment and human being. For example, we must follow the laws concerning environmental protection such as xe2x80x9cPollutant Release and Transfer Registerxe2x80x9d (PRTR Law) in Japan. Therefore, in order to protect environment, it is needed that halogen-containing solvents are replaced with other solvents including no halogen atom when charge transport layers are formed.
Among solvents including no halogen atom for use in charge transport layer coating liquids, tetrahydrofuran is promising because the resultant photoreceptor has better charge properties than that prepared by coating liquid including a halogen-containing solvent when the charge transport layer coating liquids are coated after preserved for a long period of time. However, tetrahydrofuran is mixed with water at any mixing ratio. Therefore when a coating liquid including tetrahydrofuran is used for forming a charge transport layer, the resultant charge transport layer tends to change the crystal form of the TiOPc included in the charge generation layer which contacts the charge transport layer, resulting in deterioration of the photosensitivity of the photoreceptor.
Because of these reasons, a need exists for a photoreceptor having high sensitivity and good charge stability even when repeatedly used for a long period of time.
Accordingly, an object of the present invention is to provide a photoreceptor having a good combination of high sensitivity and good charge stability even when the photoreceptor is repeatedly used for a long period of time.
Another object of the present invention is to provide a method for manufacturing the photoreceptor of the present invention.
Yet another object of the present invention is to provide an electrophotographic image forming method and apparatus (including a process cartridge) in which good images can be stably produced without causing undesired images even when images are repeatedly produced for a long period of time.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a photoreceptor including an electroconductive substrate and a photosensitive layer which is formed on the substrate and which includes at least a charge generation layer and a charge transport layer including at least a charge transport material, a binder resin, water and tetrahydrofuran (hereinafter referred to as THF), wherein the charge generation layer includes a titanyl phthalocyanine crystal which has an X-ray diffraction spectrum such that a maximum diffraction peak is observed at a Bragg (2xcex8) angle of 27.20xc2x0xc2x10.20xc2x0 when a specific X-ray of Cu-Kxcex1(wavelength of 1.542 xc3x85) irradiates the titanyl phthalocyanine crystal, and wherein a weight ratio, water/THF, in the charge transport layer is from 1/50 to 1/0.5. The weight ratio is preferably from 1/20 to 1/0.8 and more preferably from 1/10 to 1/1.
The content of THF in the charge transport layer (i.e., a weight ratio of THF to the total solid components of the charge transport layer) is preferably from 0.01 to 0.5% by weight.
The binder resin in the charge transport layer preferably includes a bisphenol-Z-form polycarbonate resin.
In another aspect of the present invention, a method for manufacturing a photoreceptor is provided which includes the steps of forming a charge generation layer including a TiOPc overlying an electroconductive substrate; and coating a coating liquid including a binder resin, a charge transport material, water and THF to form a charge transport layer on the charge generation layer, wherein the TiOPc has an X-ray diffraction spectrum such that a maximum diffraction peak is observed at a Bragg (2xcex8) angle of 27.2xc2x0xc2x10.2xc2x0 when a specific X-ray of Cu-Kxcex1 (wavelength of 1.542 xc3x85) irradiates the titanyl phthalocyanine crystal, and wherein a weight ratio, water/THF, of the water to the THF included in the charge transport layer is from 1/50 to 1/0.5.
The content of the water included in the charge transport layer coating liquid is preferably from 0.1% to 4.0% by weight
In yet another aspect of the present invention, an electrophotographic image forming method is provided which includes the steps of charging a photoreceptor, irradiating the photoreceptor with imagewise light to form an electrostatic latent image thereon, developing the latent image with a toner to form a toner image, transferring the toner image to a receiving material, cleaning the photoreceptor after the image transferring, and discharging the residual potential of the photoreceptor after the cleaning, wherein the photoreceptor is the photoreceptor of the present invention mentioned above.
In a further aspect of the present invention, an electrophotographic image forming apparatus is provided which includes the photoreceptor of the present invention mentioned above, an image irradiator, an image developer, an image transfer, a cleaner and a discharger. A process cartridge including at least a housing and the photoreceptor of the present invention is also provided.